Enthalpy is a useful function for two reasons. First, the expression U + PV occurs often in thermodynamics, and it is convenient to define this quantity. Second, if a process begins and ends at the same pressure, the amount of energy exchanged with the environment as heat is given by the change in the system's enthalpy.

An enthalpy change (ΔH) in chemistry describes the heat energy exchange with the surroundings at constant pressure. It can provide a useful guide to the feasibility of a reaction taking place.